Authentication And Key Agreement Protocol Research Articles

A deep learning-based authentication protocol for IoT-enabled LTE systems

The connected devices in Internet-of-Things (IoT)-enabled systems are continuously increasing nowadays, and likely to grow exponentially worldwide in near future. Hence, the next generation IoT-enabled mobile networks (e.g., 5G onward) are expected to provide higher system capacity and ultra-low latency to deal with. According to the Third Generation Partnership Project (3GPP), Long-Term Evolution (LTE) technology can serve the purpose efficiently, and also bridge the gap between earlier and future generation mobile networks. However, the network may face problems associated with privacy and security, as the underlying communication is mostly wireless. Thus, a secure and efficient Authentication and Key Agreement (AKA) protocol is desirable. Recently, many protocols have been proposed to address these goals. Unfortunately, the security and efficiency of such protocols are still in doubt. This paper introduces a deep learning-based AKA protocol for IoT-enabled LTE systems. The proposed protocol can address mutual authentication among the communicating entities. It employs a Deep Residual Network (DRN)-based key generation technique, called DRN-KeyGen, to establish a shared secret key on-the-fly among the communicating entities in such environment where the number of IoT devices are excessively large and extremely heterogeneous in nature. The security of DRN-KeyGen is verified considering various active and passive attacks that may occur in wireless-enabled LTE systems. The efficiency of DRN-KeyGen is measured through two different parameters: attack detection rate and attack detection time, where each attack is experimented using Python tool-based simulation with varied key lengths. The empirical results show that the proposed DRN-KeyGen can achieve an average detection rate of 0.924 and an average detection time of 30.634 s considering key lengths of 32 bits, 64 bits, 128 bits, 192 bits, 256 bits, and 512 bits. Finally, we have compared the security and efficiency of DRN-KeyGen with other existing protocols to show its superiority.

FVF-AKA: A Formal Verification Framework of AKA Protocols for Multi-server IoT

As IoT in a multi-server environment increases resources’ utilization, more and more problems of IoT authentication and key agreement are being revealed. The Authentication and Key Agreement (AKA) protocol plays an important role in solving these problems. Many AKA protocols have been proposed, and some of them support their own verifications. However, a unifying verification framework for multi-server IoT is lacking. In this article, we propose a formal verification framework of AKA protocols for multi-server IoT (FVF-AKA). It supports the construction of CSP models for the AKA protocol, the implementation of the CSP models in PAT with C#, and the verification of formal models. With the help of C#, many complex functions in the AKA protocol can be implemented. We also design an algorithm to support automatic conversion from the CSP model to the PAT model. FVF-AKA can verify four fundamental properties (deadlock freedom, entity legitimacy, timeout delay, and session key consistency). It also supports the verification of security properties for the AKA protocol suffering from four different attacks (relay attacks, denial of service attacks, server spoofing attacks, and session key attacks). Our approach can be applied to most AKA protocols for multi-server IoT generally. By applying FVF-AKA to two AKA protocols, we can verify whether they satisfy the fundamental properties and analyze their security properties in vulnerable environments. Our work would help to analyze the AKA protocol for multi-server IoT and provide the foundation for the analysis of enhancing its security and robustness.

Two protocols for improving security during the authentication and key agreement procedure in the 3GPP networks

In the fifth generation of cellular networks (5G), data transmission with the highest possible speed and the lowest latency is one of the most essential 5G designing criteria. Furthermore, this generation is supposed to add new applications, features, and services to mobile phone networks with a high commitment to security and privacy. Hence, primitive protocols proposed for the 5G networks generally should be able to achieve the security requirements and also have low network overheads. In this paper, we focus on improving two primitive authentication and key agreement (AKA) protocols (5G AKA and EAP-AKA’). Our two improved protocols meet the security and performance requirements along with proper compatibility with the 3GPP security architecture. The paper’s security approach is to first construct a fixed-length key derivation scheme and show that it is strongly unforgeable under the adaptive chosen-ciphertext attack. Then, we demonstrate that, unlike the 3GPP AKA protocols, the message authentication codes (MACs) and session keys used in the improved protocols are also strongly unforgeable under the adaptive chosen-ciphertext attack. Moreover, we show that the improved protocols achieve some security requirements, such as mutual authentication, secure key agreement, and forward and backward secrecy of session keys. Furthermore, we explain informally that the improved protocols achieve other security requirements, such as following the 3GPP security architecture and resistance against known attacks. Finally, we compare the improved protocols with other AKA protocols regarding communication, computational, and storage overheads. This comparison shows that the improved protocols have appropriate overheads among other AKA protocols.

Provably Secure ECC-Based Authentication and Key Agreement Scheme for Advanced Metering Infrastructure in the Smart Grid

Advanced metering infrastructure (AMI) is a vital component of the smart grid (SG) for real-time data access and bidirectional communication. An authentication and key agreement (AKA) protocol is needed for AMI systems to ensure the confidentiality and integrity of communication data. Since the devices are connected to the open network and generally deployed outdoors with limited computation, communication, and storage, designing a suitable AKA protocol is a challenging task. Researchers are still looking for good ways to make the SG secure and efficient simultaneously. To remedy the situation, in this article, we advance a security-enhanced elliptic-curve-cryptography-based AKA protocol, and the security has been proven rigorously under the random oracle model and verified with the ProVerif tool. Furthermore, performance comparison validates the proposed protocol in affording improved security features with lower computation and communication cost. In addition, the proposed scheme is implemented practically on a testbed, which is deployed using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Raspberry Pi 3 Model B+</i> for smart meters.

The vulnerability and enhancement of AKA protocol for mobile authentication in LTE/5G networks

The Long-Term Evolution (LTE)/5G network connects much of the world’s population to provide subscriber’s voice calls and mobile data delivery, with security provided by the Authentication and Key Agreement (AKA) defined by 3GPP, which makes the LTE/5G network more secure than all its predecessors. Primarily due to the access limitations of LTE systems, the vulnerabilities of AKA protocol and potential attacks have not received much investigation, which is essential to LTE users with a tremendous amount of cellular services. In this study, we focus on two questions: (i) what are the vulnerabilities that can be exploited to carry out attacks in practice? and (ii) how to design an enhanced AKA protocol against such attacks? We examine the detailed procedures of Evolved Packet System (EPS)-AKA protocol by 3GPP, and have identified three types of attacks with respect to catching, location tracking, and jamming. We have designed and implemented attacks with commercial equipment to evaluate their threats in practice. In addition, we propose an enhanced AKA protocol that essentially relies on asymmetric encryption rather than symmetric in the AKA protocol and additional digital signatures to countermeasure these attacks. Finally, we verified our solution through formal verification to prove that our solution can mitigate the newly found vulnerabilities.

Secure Long-Range Autonomous Valet Parking: A Reservation Scheme With Three-Factor Authentication and Key Agreement

Long-range autonomous valet parking (LAVP) is a current trend, partly due to traffic congestion and parking headache. For large-scale parking demands, reservation is introduced to effectively manage valet parking. However, existing schemes focus on parking request verification and parking check-in, which aren't applicable to LAVP because they ignore identity legitimacy and communication security in the phase of picking up as well as dropping off passengers. One viable solution is authentication and key agreement (AKA) protocol. Generally, due to low entropy of passwords and dictionary attacks, three-factor (i.e. passwords, biometrics, and smart card) AKA is more secure than single- and two-factor AKA. Unfortunately, known attacks and high overheads hinder the application of three-factor AKA in real-world environments. Hence, one of the most tough tasks is to balance security and availability, especially how to address the potential threats introduced by each factor while taking full advantage of three factors. Inspired by the above challenges, we propose a provably secure three-factor AKA protocol for reservation services in LAVP, namely SecLAVP. Specifically, the passenger and the autonomous vehicle ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$AV$</tex-math></inline-formula> ) complete mutual authentication with the assistance of drop-off/pick-up point ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$DP$</tex-math></inline-formula> ). After successful authentication, the session key is generated between the passenger, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$DP$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$AV$</tex-math></inline-formula> for secure communication. In the Real-Or-Random (ROR) model, we formally prove SecLAVP satisfies session-key security. Then, we apply AVISPA to simulate our proposed protocol, to demonstrate that SecLAVP can resist man-in-the-middle attacks. Additionally, informal security analysis indicates that SecLAVP satisfies our defined 16 design goals concerning security. Finally, we evaluate performance of SecLAVP in terms of communication overheads, computational overheads, and scheduling, to manifest the feasibility.

Formalization and evaluation of EAP-AKA’ protocol for 5G network access security

The end user’s Quality of Experience (QoE) will be improved while accessing services in Fifth Generation Mobile Network (5G), supported by enhanced security and privacy. The security guarantees offered by the Authentication and Key Agreement (AKA) protocols will be depended upon by end users and network operators. The AKA protocols have been standardized for 5G networks, and the Extensible Authentication Protocol (EAP)-AKA’ protocol is one of the main authentication mechanisms that has been specified for User Equipment (UE) and network mutual authentication. This article models the EAP-AKA’ protocol and conducts an extensive formal verification of the EAP-AKA’ protocol as defined in the 5G security standard to determine whether the protocol is verifiably secure for 5G. It provides a security evaluation of the EAP–AKA’ protocol based on the current 5G specifications using ProVerif, a security protocol proof verifier. It also presents security properties that support the security verification, as well as quantitative properties that are used to assess the protocol’s performance. Finally, it compares the EAP-AKA’ and 5G-AKA protocols’ security and performance results.

ISG-SLAS: Secure and lightweight authentication and key agreement scheme for industrial smart grid using fuzzy extractor

Smart grid (SG) has been received significant attention due to various services such as renewable energy and demand response. However, SG is fragile to various security attacks because an adversary can eavesdrop, insert, delete, or intercept the exchanged messages over an open channel. Thus, secure and lightweight authentication and key agreement (AKA) scheme for SG to ensure the necessary security requirements. Recently, the existing schemes designed a secure and efficient AKA protocol for industrial SG using cryptographic primitives to ensure reliable energy services. However, we prove that existing schemes for industrial SG are vulnerable to potential security attacks and do not guarantee the necessary security requirements. To enhance the security flaws of the existing schemes, we design a secure and lightweight AKA scheme for SG using fuzzy extractor, called ISG-SLAS. ISG-SLAS resists various security attacks and provides security functionalities. We evaluate the security of the ISG-SLAS using ROR model and AVISPA simulation. Moreover, we present the testbed experiments using MIRACL based on Raspberry PI 4. We then present a comparative analysis between ISG-SLAS and related schemes. Consequently, ISG-SLAS better ensures security and efficiency compared with related schemes and is more suitable than related schemes for practical SG.

PUF-Based Authentication and Key Agreement Protocols for IoT, WSNs, and Smart Grids: A Comprehensive Survey

Physically unclonable function (PUF) is a physical unit fabricated inside a sensor and generally considered as an assurance anchor of resource inhibited device. Essentially, the function is based on the cryptographic approach, where a key is created and utilized such that it cannot be cloned. More specifically, it is an arbitrary function, which maps inherent properties of the hardware devices to a unique bit stream of information. Authentication and key agreement (AKA) protocols are widely used in electronic commerce, electronic stock trading, and many secured business transaction platforms, because they allow the communicating devices to mutually authenticate each other while exchanging authenticated session key (or secret key) that can be used subsequently to establish a secured communication channel. Yet, these protocols are also vulnerable to a broad range of security outbreaks. In light of these notions and practical applications, this article is intended to: 1) provide an overview of AKA protocols, PUF plus the combined PUF-based AKA; 2) systematically and taxonomically examine and discuss with pros and cons of AKA applications to the fast growing areas of Internet of Things, wireless sensor networks, and smart grids based on a meticulous survey of the existing literature; 3) summarize the challenges to deployment and potential security risks of the underlying technologies and possible remedies or mitigation strategies; and 4) to conduct and report a comparative performance and security analysis with respect to the three focused areas.

5GAKA-LCCO: A Secure 5G Authentication and Key Agreement Protocol with Less Communication and Computation Overhead

There are still some shortcomings in the latest version of the 5G authentication and key agreement (AKA) protocol, which is specified by the third-generation partnership project (3GPP). To overcome these shortcomings, an improved primary authentication and key agreement protocol for 5G networks (5G-IPAKA) were proposed. However, one of the shortcomings of the 5G AKA protocol has not been completely overcome in the 5G-IPAKA protocol, resulting in denial of service (DoS) attacks against both the serving network (SN) and the home network (HN). In addition, the 5G AKA protocol has large communication and computation overhead, while the 5G-IPAKA protocol has an even larger communication and computation overhead. These will lead to a great deal of energy consumption. To solve these problems, a secure 5G authentication and key agreement protocol, with less communication and computation overhead (5GAKA-LCCO) is proposed. Then, the 5GAKA-LCCO protocol is proven secure in both the strand space model and the Scyther tool. Further discussion and comparative analysis show that the 5GAKA-LCCO protocol can completely overcome the shortcomings of the latest version of the 5G AKA protocol and is better than the recently improved 5G AKA protocols in overcoming these shortcomings. Additionally, the 5GAKA-LCCO protocol has less communication and computation overhead than the 5G AKA protocol and the recently improved 5G AKA protocols.

An efficient two‐factor authentication protocol based on elliptic curve cryptosystem with provable security

AbstractWith the gradual penetration of computers and communication networks into people's lives, more and more people begin to pay attention to the information security. Key agreement protocol provides identity authentication for participants in communication system and generates a temporary session key for participants to encrypt messages. Recently, Wang et al designed an authentication and key agreement (AKA) protocol based elliptic curve cryptosystem and proved that their scheme can resist various known attacks. In this article, we analyze the security of their scheme and demonstrate that it is vulnerable to the temporary factor leakage attacks and insider attacks. Then, based on the elliptic curve public key cryptographic algorithm, we design a secure anonymous two‐factor user AKA protocol. And we use the well‐known random oracle model (ROM) and traditional heuristic discussion to prove the security of the proposed protocol. By comparing with several existing protocols based on public key cryptography algorithm, it is found that this protocol is more secure and efficient. The security analysis shows that the protocol designed in this article can meet all kinds of security requirements and consumes the less computing resources on users and servers, and is especially suitable for the scenarios with high security and efficiency requirements.

Construction of elliptic curve cryptography‐based authentication protocol for internet of things

AbstractThe advancement in the internet of things (IoT) is bringing changes in our daily lives. The widespread adoption of the IoT in a variety of areas sets the basis for smart living while also posing privacy and security concerns. Authentication and key agreement (AKA) protocol provides identity authentication for participants in a communication system and generates a temporary session key for participants to securely transmit messages. In addition to this, the adoption of smart cards adds a second line of defense in AKA protocols for IoT. To enhance the security of AKA protocols for IoT, many AKA protocols are proposed, but the majority of them fail to manage the balance between security and efficiency. To achieve both security and efficiency together for IoT, based on the elliptic curve public‐key cryptography, we design a three‐factor AKA protocol. We follow security analysis and efficiency analysis to analyze the security and efficiency. By comparing with related existing protocols, it is found that this protocol achieves security and efficiency attributes. The investigation shows that the protocol can meet key security requirements, consumes less computing resources, and is especially suitable for IoT scenarios with high security and efficiency.

5G-IPAKA: An Improved Primary Authentication and Key Agreement Protocol for 5G Networks

The 3rd generation partnership project (3GPP) has been enhancing the security of the 5G AKA (authentication and key agreement) protocol. However, there may still be some shortcomings in the latest version of the 5G AKA protocol. According to the analysis of the latest version of the 5G AKA protocol, this paper points out seven of its shortcomings. To overcome these shortcomings, an improved primary authentication and key agreement protocol for 5G networks is proposed, which is named 5G-IPAKA. Compared with the latest version of the 5G AKA protocol, the main improvements include that the pre-shared key between the user equipment (UE) and the home network (HN) is replaced with a derivation key as the pre-shared key, the challenge-–response mechanism for the serving network (SN) is added, the mutual authentication and key confirmation occurs between the UE and the SN, and the message authentication code (MAC) failure procedure is replaced with a timeout mechanism on the HN. Then, the 5G-IPAKA protocol is proven secure in the mixed strand space model for mixed protocols. Further discussion and comparative analysis show that the 5G-IPAKA protocol can overcome the above shortcomings of the latest version of the 5G AKA protocol, and is better than the recently improved 5G AKA protocols. Additionally, the 5G-IPAKA protocol is efficient and backward-compatible.

A Secure Anonymous D2D Mutual Authentication and Key Agreement Protocol for IoT

Internet of Things (IoT) is a developing technology in our time that is prone to security problems as it uses wireless and shared networks. A challenging scenario in IoT environments is Device-to-Device (D2D) communication where an authentication server, as a trusted third-party, does not participate in the Authentication and Key Agreement (AKA) process and only cooperates in the process of allocating and updating long-term secret keys. Various authentication protocols have been suggested for such situations but have not been able to meet security and efficiency requirements. This paper examined three related protocols and demonstrated that they failed to remain anonymous and insecure against Key Compromise Impersonation (KCI) and clogging attacks. To counter these pitfalls, a new D2D mutual AKA protocol that is anonymous, untraceable, and highly secure was designed that needed no secure channel to generate paired private and public keys in the registration phase. Formal security proof and security analysis using BAN logic, Real-Or-Random (ROR) model, and Scyther tool showed that our proposed protocol satisfied security requirements. The communication and computation costs and energy consumption comparisons denoted that our design had a better performance than existing protocols.

SEAI: Secrecy and Efficiency Aware Inter-gNB Handover Authentication and Key Agreement Protocol in 5G Communication Network

Recently, the Third Generation Partnership Project (3GPP) has initiated the research in the Fifth Generation (5G) network to fulfill the security characteristics of IoT-based services. 3GPP has proposed the 5G handover key structure and framework in a recently published technical report. In this paper, we evaluate the handover authentication mechanisms reported in the literature and identify the security vulnerabilities such as violation of global base-station attack, failure of key forward/backward secrecy, de-synchronization attack, and huge network congestion. Also, these protocols suffer from high bandwidth consumption that doesn’t suitable for energy-efficient mobile devices in the 5G communication network. To overcome these issues, we introduce Secrecy and Efficiency Aware Inter-gNB (SEAI) handover Authentication and Key Agreement (AKA) protocol. The formal security proof of the protocol is carried out by Random Oracle Model (ROM) to achieve the session key secrecy, confidentiality, and integrity. For the protocol correctness and achieve the mutual authentication, simulation is performed using the AVISPA tool. Also, the informal security evaluation represents that the protocol defeats all the possible attacks and achieves the necessary security properties.Moreover, the performance evaluation of the earlier 5G handover schemes and proposed SEAI handover AKA protocol is carried out in terms of communication, transmission, computation overhead, handover delay, and energy consumption. From the evaluations, it is observed that the SEAI handover AKA protocol obtains significant results and strengthens the security of the 5G network during handover scenarios.

An efficient three-factor remote user authentication protocol based on BPV-FourQ for internet of drones

Due to the mobility, flexibility and autonomy of drones, the Internet of Drones (IoD) have gained momentum recently and are being widely used in a variety of fields, such as agriculture, industry, and transportation. However, due to the intrinsic openness of wireless communication channels in IoD, the data generated by drones are facing new security challenges and privacy issues. To ensure the confidentiality of data communication, it is crucial to authenticate remote users and establish session keys for IoD through the authentication and key agreement (AKA) protocol. Unfortunately, most existing AKA protocols using asymmetric cryptographic primitives are computationally expensive, and are unaffordable for the computing power limited drones. To this end, this paper presents an efficient three-factor AKA protocol for IoD based on FourQ and Boyko-Peinado-Venkatesan (BPV) pre-calculation. The detailed security analysis reveals that our protocol is secure against known attacks and achieves important security goals, especially perfect forward secrecy. Besides, by performing real world experiments on the Raspberry Pi, we demonstrate that the improved FourQ curve primitive is 4–5 times faster than the conventional elliptic curve primitives. The security and performance comparison are also given to prove that the proposed protocol provides stronger security and higher efficiency than existing ones.

A Novel Group-based Secure Lightweight Authentication and Key Agreement Protocol forMachine-Type Communication

Nowadays, one of the most important criterions in designing different generations of cellular technology is to handle a large number of heterogeneous devices with high security guarantees. The first significant security issue considered in this field is mutual authentication of the devices and the network and authenticated key agreement between them. Hence, various authentication and key agreement (AKA) protocols were proposed for Long Term Evolution (LTE) and 5G networks. However, each of the protocols suffer from various security and performance problems. This paper proposes a group-based secure lightweight authentication and key agreement (GSL-AKA) protocol for machine-to-machine (M2M) communication. Security analysis and formal verification using the AVISPA tool prove that the proposed protocol overcomes various known security attacks and provides all the considered security requirements. Moreover, performance analysis shows that the communication and computation overheads of the proposed protocol are the lowest in comparison with the other existing group-based AKA protocols.

Group authentication protocol based on aggregated signatures for D2D communication

Device-to-device (D2D) communication is one of the most promising technologies of new mobile communication networks (5G), and essential for the Communicating Things Networks (CTNs) paradigm. Its application scope has been widened and billions of devices are expected to be communicating in the next years. Moreover, goals, such as end-to-end security and reductions in computational costs required by resource-constrained devices, must be accomplished for its full implementation. The Third Generation Partnership Project (3GPP) standardized authentication and key agreement (AKA) protocols are not suitable for D2D due to differences in the architecture and communication scenario. They may cause several security issues and excessive computational and transmission overhead, since they require individual executions for each pair of devices. The development of AKA protocols adapted to D2D group communication is still in its initial steps. This article introduces a new authentication and key agreement protocol for D2D groups of devices based on asymmetric cryptography and aggregated signatures. The scheme focuses on robust security and reductions in computational and communication costs. It was compared to other group AKA protocols and yielded better results regarding security and overhead reduction. A formal validation conducted by Automated Validation of Internet Security Protocols and Applications tool (AVISPA) proved its robustness.

Practical and Provably Secure Three-Factor Authentication Protocol Based on Extended Chaotic-Maps for Mobile Lightweight Devices

Due to the limitations of symmetric-key techniques, authentication and key agreement (AKA) protocols based on public-key techniques have attracted much attention, providing secure access and communication mechanism for various application environments. Among these public-key techniques used for AKA protocols, chaotic-map is more effective than scalar multiplication and modular exponentiation, and it offers a list of desirable cryptographic properties such as un-predictability and higher efficiency than scalar multiplication and modular exponentiation. Furthermore, it is believed that three-factor AKA protocols can achieve higher security level than single- and two-factor protocols. However, none of existing three-factor AKA protocols can meet all security requirements. One of the most prevalent problems is how to balance security and usability. To deal with this problem, we put forward a provably secure three-factor AKA protocol based on extended chaotic-maps for mobile lightweight devices, by adopting the techniques of Fuzzy-Verifiers and Honeywords. We prove the security of our protocol in the random oracle model. We also simulate the protocol by using the AVISPA tool. The security analysis and simulation results show that our protocol can meet all 13 evaluation criteria regarding security. The evaluation results demonstrate that our protocol offers better balance between security and usability over state-of-the-art ones.

A Blockchain-Based Authentication and Key Agreement (AKA) Protocol for 5G Networks

Subscriber authentication is a primitive operation in mobile networks required by each operator prior to offering any service to end users. In this paper, we propose a novel blockchain-based Authentication and Key Agreement (AKA) protocol for roaming services in 5G networks. Each Home Network (HN) creates its own smart contract and publishes its address to inform other operators who want to offer roaming services to HN subscribers. All subsequent communication between the HN and Serving Network (SN) is done by calling the function of this smart contract. The proposed protocol eliminates the need for a secure channel between the HN and SN, which is a primary requirement of current 5G AKA protocols. In practice, a secure channel requires the HN and SN to establish a secure session before running the AKA protocol. Further, the proposed protocol leverages the benefits of blockchain, such as auditable log, decentralized architecture, and the prevention of Denial of Service (DoS) attacks. Furthermore, we provide a security proof of the protocol through formal verification using ProVerif. The results show that our scheme tends to preserve user privacy and at the same time provides mutual authentication of the participants. Finally, our evaluation of the Ethereum blockchain shows that the protocol is efficient in terms of both transaction and execution costs.